Insert bearing shells have been used in numerous bearing applications including, but not limited to, engine main and connecting rod bearings, railway vehicle axle bearings and engine piston pin bearings of various types. In the particular case of engine piston pin bearings, a commonly used application has been as the main load carrying bearing in the oscillating piston pin joint of two-cycle engine pistons. Pertinent examples of the latter include U.S. Pat. Nos. 3,433,539 Nigh, 3,555,972 Hulsing and 3,762,389 Malina, all assigned to the assignee of the present invention.
In these two-stroke cycle engine piston applications, an insert bearing shell is received within a recess on the main thrust receiving side of a piston pin receiving bore in a member of the two-stroke cycle piston assembly. Bearing inserts are generally not needed in the opposite portions of the bore adjacent the connecting rod, since reverse loading of the piston pins in two-stroke cycle engines is seldom encountered or is of negligible value. For this reason also, the piston pin bore in such piston assemblies can be made with relatively large clearance around the piston pin portions which do not engage the bearing insert.
In four-stroke cycle engine pistons, the operating conditions may impose significant reverse loads on the piston pins during the piston intake and exhaust strokes when outward centrifugal forces are not significantly counteracted, and may be added to, by low or below ambient cylinder pressures acting on the pistons. Thus, in four-stroke cycle piston assemblies, load carrying bearings are commonly required on both the top and bottom sides of the piston pin. Known constructions include the use of individual bushings as in U.S. Pat. Nos. 2,850,340 Brill and 3,943,908 Kubis et al. A one piece bushing with extended upper surface is shown in U.S. Pat. No. 2,069,594 Schneider. These arrangements not only provide load carrying capability in both directions of piston reciprocation, but also provide a close piston pin to bearing clearance that is desirable to minimize the shock loads from changes in load direction occurring in the operation of four-stroke cycle engine pistons.
Past attempts to apply the advantages of the two-stroke cycle piston slipper-type less than semicylindrical insert bearing technology to four-stroke cycle engines have been impaired by relatively loose manufacturing tolerances required on the bearing inside diameter and bearing crush. Currently, such slipper-type bearings with arc lengths less than 180.degree. are dimensioned on an arc-length basis. Thus, the arcuately opposite edges of the bearing shell and the cooperating edges of the recess in which the shell is received generally define surfaces lying in planes that extend radially from the center of the arc formed by the piston pin bearing. As a result, required manufacturing tolerances make it difficult to obtain, in a practical way, adequately close diametral tolerances between oppositely placed slipper bearing inserts for engaging the piston pin of a four-stroke cycle engine piston.